This invention relates generally to syringes and, more particularly, to syringes for dispensing fluids that are frozen for storage and then thawed from the frozen state, along with the syringe, shortly before use.
Syringes containing a frozen fluid, such as premixed curable liquids, epoxies, pastes, encapsulants, underfills, dam fluids, thixotropic liquids, and adhesives, are used in semiconductor and optoelectronic packaging, as well as numerous other applications. Premixing substances like adhesives and pastes eliminates handling and weighing individual components, the mess associated with mixing, testing before and after mixing for quality assurance, and special handling problems. Syringes are also referred to by other conventional terms such as cartridge, barrel, tube, or reservoir.
The syringe is filled with fluid at room temperature and frozen at a temperature below the fluid's freezing point. The syringe and its frozen contents are shipped in a thermally insulated container from the manufacturer at a temperature of about −40° C. to about −80° C. (i.e., on dry ice). The end user stores the frozen syringe and fluid in a low-temperature freezer at a temperature below the fluid's freezing point. Low temperature storage extends the working life of the fluid, delays aging and prevents curing, which cooperate to extend or prolong the fluid shelf life. Shortly before dispensing the fluid from the syringe, the end user warms the syringe and fluid to ambient temperature, which serves to thaw the frozen fluid. After the fluid is dispensed, the syringe is discarded.
When thawed from the frozen state, conventional syringes containing such frozen fluids are susceptible to a phenomenon known as freeze-thaw voiding. Specifically, the frozen fluid and the syringe each shrink in dimensions or contract when frozen. It is believed that, because the coefficients of thermal expansion of the solid polymer material forming the syringe and the fluid differ, the degree of dimensional shrinkage differs. When warmed, the sidewall of the syringe warms at a faster rate than the frozen mass of fluid held inside the syringe, which causes the sidewall to expand before the frozen fluid. The sidewall is believed to pull away from the frozen fluid to define air-filled spaces between the frozen mass of fluid and the sidewall. As the fluid thaws and randomly re-wets regions of the syringe sidewall, the air-filled spaces are surrounded by the fluid and define air bubbles or pockets near the sidewall. The trapped air pockets, termed by some as freeze-thaw voids, adversely impact the dispensability of the fluid from the syringe. Specifically, dispensing fluid laced with air-filled voids causes dispensing inconsistencies including but not limited to tailing, dripping, dispense voids, and weight variations.
For some fluids, freeze-thaw voids are observed to form in the vast majority of fluid-filled syringes. Depending upon the fluid type, the voids may spontaneously alleviate by migrating to the vicinity of the syringe plunger and perhaps passing rearwardly between the periphery of the plunger and the bore of the barrel. For other fluid types, the voids remain stationary and, therefore, are not self-alleviating. In this circumstance, one option available to the end user is to discard the unused syringe to avoid the potential for dispensing fluid containing the voids. Another option for the end user is to cautiously dispense the fluid from the syringe without dispensing the voids. In the latter option, only a portion of the fluid is dispensed and the syringe is ultimately discarded with unused fluid remaining inside the syringe.
The industry has tried without success for years to mitigate the effect of voids that are not self-alleviated by controlled warming of the fluid and syringe. After removal from the freezer, a common approach for controlled warming is to place the frozen, filled syringes inside a thermally insulated blanket or sleeve, usually composed of a foam material. The thermally insulated sleeve slows the rate at which the frozen fluid and syringe warm toward ambient temperature. However, this conventional approach has only been moderately successful in eliminating freeze-thaw voids. Moreover, the thermally insulating sleeve needlessly consumes extra space inside the thermally insulated container used by the manufacturer to ship the syringes. If not recycled, the thermally insulating sleeve must be disposed of after use. In addition, the sleeve may not securely grip and hold the syringe, which creates the potential for the syringe to fall out of the sleeve.
In view of these and other difficulties associated with freeze-thaw voiding, it would therefore be desirable to provide a syringe for fluids that reduces or minimizes the incidence of freeze-thaw voiding.